This invention relates to a method of, apparatus and a printing plate for printing raised ink images, and even more specifically for direct printing raised ink hectographic or spirit duplicating masters.
Hectographic or spirit duplicating is a widely used process and is especially attractive because of its low per copy and equipment costs and because no special training is required to operate or service the equipment. Generally, a spirit duplicating master includes a substrate, usually paper, carrying a hectographic ink mirror image of the material to be duplicated. The hectographic ink on the master contains a spirit or alcohol soluble dye which when pressed into momentary printing contact with a sheet of paper which has been previously wetted with a spirit or alcohol solution transfers dye from the master to the sheet. It can thus be seen that some of the hectographic ink is used up from the master as each hectographic copy is printed and that in order for a hectographic master to have a relatively long copy run life and to print legible copies, it must have a relatively thick hectographic ink image thereon.
In many instances, it is desirable to print large numbers of identical pre-printed hectographic masters. Conventionally, hectographic masters are printed by either of four methods.
First and most commonly used is the so-called impact or crash printing method which uses a master set comprising a master substrate sheet and an ink sheet having a solid coating of hectographic ink thereon, the ink sheet being in face-to-face contact with the substrate. The master set is printed on a conventional letter press by passing the set between the nip of the plate cylinder having a raised letter plate thereon and the impression roll of the press so that the raised printing areas on the plate exert great pressure on the master set to thus forceably press the substrate into the ink sheet and to transfer ink to the substrate to form a mirror ink image on the substrate. Since the ink on the ink sheet is relatively thick, a heavy or raised ink image is formed on the substrate so that crash printed hectographic masters have a relatively long copy run life. While the crash printing method has the advantage that the ink sheet can be retained as a part of a master set so that a user can add other information to the master prior to duplicating, this crash printing method of making hectographic masters is expensive because an ink sheet must be provided for each master. Also, because the pressure required to transfer the hectographic ink from the ink sheet to the substrate must be transferred through the sheets of the master set, it is difficult to print masters having sharply defined characters or large solid areas to be printed.
The second method of printing hectographic masters utilizes a rotary letter press to direct print a mirror hectographic ink image on a master substrate. While this method eliminates the requirement of providing an ink sheet for each master and is thus of much lower cost, and while this method also provides a master with more sharply defined ink images, masters produced by this direct printing method have a relatively short copy run life and often do not print clear, legible copies. Letter press printing generally requires the use of a low viscosity ink. Usually, these low viscosity inks set by solvent evaporation and/or by absorption of the solvent into the copy paper. Also, letter press plates carry a relatively thin layer of ink on their printing surfaces. The solvent loss of the ink and the relatively thin layer of ink carried by the letter press printing plate together result in a relatively thin layer of ink remaining on the master substrate and that in turn results in a relatively low copy run life for masters printed by the direct print method.
The third known method of making identical hectographic masters is that of gravure direct printing utilizing engraved metal print rolls and a hectographic ink. The high cost of these engraved gravure print rolls or plates makes this method expensive for all but the longest printing runs of identical hectographic masters. This method requires a low viscosity hectographic ink which sets by solvent evaporation or absorption into the paper which in turn results in thin ink images. The thin ink images on the master in turn result in poor copy quality and short copy run life for the master.
The fourth known method of printing hectographic masters involves screen printing the ink image on a master substrate. While screen printing does result in hectographic masters having the good run life of crash printed masters and the image sharpness of direct printed masters, screen printing is slow, involves a good deal of skilled labor, and requires special equipment. Accordingly, screen printed hectographic masters are relatively expensive.
With regard to the known prior art, reference may be made to U.S. Pat. Nos. 2,456,608 and 3,891,443 which disclose printing plates having roughened ink areas. In the first-mentioned patent, type for making proofs which in turn are used to make gravure printing plates is disclosed in which metal type is cast having a crisscrossing series of elongate grooves formed therein which terminate short of the edges of the type character. These grooves prevent the ink therein from being drawn out or dislodged by the action of the doctor blade on the printing press when the proof characters are being printed by the type. In the second-mentioned patent, a photosensitive relief printing plate is disclosed in which the surface of the plate is roughened to increase its ink holding capability either by embossing the photosensitive material prior to exposing it by running the plate between a pair of roughened nip rolls thereby to emboss a mat finish on the plate or by providing a preroughened metal plate so that the photosensitive material conforms to the roughened surface of the plate.